Fig. 387.—Map to show the cuestas which have played so important a part in fixing the boundaries of the Lake basins, and also the principal preglacial rivers by which they have been trenched (based upon a map by Grabau).
When the elevation of the land in the vicinity of this shore had caused a recession of the waters, there was formed a coastal plain on the borders of the oldland like that which is now found upon our Atlantic border between the Appalachians and the sea ([Fig. 272], [p. 246]). The rivers from the oldland cut their way in narrow trenches across the newland, and because of the harder limestone formations, their tributaries gradually became diverted from their earlier courses until they entered the trunk stream nearly at right angles and produced the type of drainage network which is called “trellis drainage.” It is characteristic of this drainage that few tributaries of the second order will flow up the natural slope of the beds, but on the contrary these natural slopes are followed in the softer rock nearly at right angles again to the tributaries of the first order of magnitude ([Fig. 387]). Thus are produced a series of more or less parallel escarpments formed in the harder rock and having at their base a lowland which rises gradually in the direction of the oldland until a new escarpment is reached in the next lower of the hard formations. Such flat-topped uplands in series with intermediate lowlands and separated by sharp escarpments are known as cuestas (see [p. 246]), and the Lewiston Escarpment limits that formed in Niagara limestone ([Figs. 387] and [388]).
Fig. 388.—Bird’s-eye view of the cuestas south of Lakes Ontario and Erie (after Gilbert).
Episodes of Niagara’s history and their correlation with those of the Glacial Lakes.—Of the early episodes of Niagara’s history, our knowledge is not as perfect as we could desire, but the later events are fully and trustworthily recorded. The birth of the Falls is to be dated at the time when the ice front had here first retired into what is now Canadian territory, thus for the first time allowing the waters from the Erie basin to discharge over the Lewiston Escarpment into the basin of the newly formed Lake Iroquois ([Fig. 364], [p. 334]). Since the level of Lake Iroquois was far above that of the present Lake Ontario, the new-born cataract was not the equivalent in height of the escarpment to-day. The Iroquois waters then bathed all the lower portion of the escarpment, so that the foot of the Fall was upon the borders of the Lake.
In order to interpret the history of the Niagara gorge, we must remember that the effective drilling of this gorge was in each stage dependent mainly upon the volume of water discharged from Lake Erie, a large discharge being recorded by a channel drilled both wide and deep, while that produced by the discharge of a smaller volume was correspondingly narrow and shallow. To-day the gorges of large cross section have, moreover, a relatively placid surface, whereas through the constricted sections the water of the river is unable to pass without first raising its level at the upper end and under the head thus produced rushing through under an increased velocity. The best illustration of such a constricted section is the Gorge of the Whirlpool Rapids.
Fig. 389.—Sketch map of the greater portion of the Niagara Gorge to show the changes in cross section in their relations to Niagara history (based upon a map by Taylor).